The objective of this research application is to unveil the molecular and cellular mechanisms by which prostate carcinoma-produced platelet-derived growth factor (PDGF) regulates osteoclastic bone resorption and new bone growth, creating a favorable microenvironment for metastatic deposit. Increasing evidence indicate the significance of PDGF receptor signaling, especially 2-PDGFR, in prostate cancer progression and bone metastasis. PDGF B, however, originally thought to be the sole ligand for 2-PDGFR, has rarely been found in prostate cancer clinical samples. Importantly, our recent immunohistochemical analysis of human prostate carcinoma specimens showed that increased expression of PDGF D, a newly discovered ligand for PDGF receptor-beta (2-PDGFR), is associated with increased Gleason scores and tumor stages. Whereas the classic PDGF ligands A and B are secreted as active dimers, PDGF D contains an N-terminal CUB domain and a C terminal PDGF domain which is secreted as a latent dimer. Extracellular proteolytic cleavage of the CUB domain is required for the PDGF domain to stimulate 2-PDGFR. Recently, we made a novel finding that human prostate carcinoma cells auto-activate latent PDGF D into an active growth factor domain of PDGF D utilizing the serine proteases uPA and matriptase. Consistent with previous reports that matriptase is frequently overexpressed in prostate cancer, especially in metastatic samples, our preliminary data showed increased matriptase expression in human prostate cancer. In a tibiae-injection model, PDGF D significantly enhanced tumor take and growth rate of LNCaP tumors with increased osteolytic and osteoblastic responses. Additionally, our in vitro study unveiled exciting new roles of PDGF D in the regulation of osteoclast differentiation independent of the RANKL/RANK signaling axis as well as osteoblast differentiation involving modulation of homeoprotein expression. Taken together, we hypothesize that prostate carcinoma-produced PDGF D, activated by the transmembrane serine protease matriptase, induces paracrine cell signaling in bone stromal cells, mediating tumor-stromal interactions critical for prostate carcinoma growth in the bone microenvironment. To test this hypothesis, we propose (1) To investigate matriptase-mediated proteolytic processing of PDGF D; (2) To investigate the molecular mechanisms by which PDGF D induces osteoclast and osteoblast differentiation using in vitro models of RAW264.7 and MC3T3-E1 cell lines; and (3) To investigate the roles of PDGF D and its functional regulator matriptase in bone stromal responses and prostate tumor growth in animals. Considering that 2-PDGFR as well as matriptase is highly upregulated in both bone metastases and primary prostate cancer specimens, the completion of the proposed study of interplay between PDGF D, a specific activator of 2-PDGFR, and matriptase will help us in understanding the molecular basis for prostate cancer progression. In addition, the proposed study may provide important information with therapeutic value, especially given that VEGF/PDGF inhibitors are currently in clinical trials for cancer patients.